1. Field of the Invention
This invention relates generally to spin valve sensors of magnetic heads, and more particularly to the use of cobalt in capping layers of spin valve sensors.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read (MR) sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which the MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g., nickel-iron, cobalt-iron, or nickel-iron-cobalt) separated by a layer of nonmagnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (AFM) layer (e.g., nickel-oxide, iron-manganese, or platinum-manganese) layer. The pinning field generated by the AFM layer should be greater than demagnetizing fields to ensure that the magnetization direction of the pinned layer remains fixed during application of external fields (e.g. fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the information recorded on the magnetic medium (the signal field). A cap or capping layer of tantalum is typically formed over the sensor stack structure for protecting the sensor during and after its production.
There are several properties of a spin valve sensor which, if improved, will improve the performance of the magnetic head and increase the data storage capacity of a disk drive. For example, it is generally desirable to increase the magnetoresistive coefficient Δr/R and decrease the coercivity Hc of the free layer without substantially increasing the thickness of the sensor layers. An increase in the spin valve effect (Δr/R) equates to higher bit density (bits/square-inch of the rotating magnetic disk) read by read head. It is also desirable to keep the magnetostriction slightly negative or zero. If the free layers structure has positive magnetostriction and is subjected to compressive stress, there will be a stress-induced anisotropy that urges the magnetic moment of the free layer from a position parallel to the ABS toward a position perpendicular to the ABS. The result is undesirable read back asymmetry and instability. The compressive stress occurs after the magnetic head is lapped at the AMS to form the strip height of the sensor. After lapping, the free layer is in compression and this, in combination with positive magnetostriction, causes the aforementioned read back asymmetry. If the free layer structure has negative magnetostriction in combination with compressive stress that the magnetic moment of the free layer is actually strengthened along the position parallel to the ABS. Thus, it is desirable that the magnetostriction of the free layer be zero or only slightly negative.
Efforts continue to improve the properties of spin valve sensors. What are needed are ways in which to increase the magnetoresisitive coefficient Δr/R, lower the coercivity Hc, and substantially eliminate magnetostriction in a spin valve sensor.